1. Field of the Invention
The present invention relates generally to a method for providing improved equalization for the reading of marks on optical data-storage media. More specifically, the invention relates to a method for digital signal processing a read signal to compensate for intersymbol interference among the marks.
2. Description of the Related Art
In order to increase the capacity and speed of optical data-storage systems, multilevel optical recording systems have been developed. Note that in this specification, the term multilevel refers to more than two levels. In a traditional optical recording system, reflectivity of the recording medium is modulated between two states. The density of data recorded on an optical recording medium can be increased by modulating the reflectivity of the optical recording medium into more than two states.
However, at high data densities, light reflected from one mark will tend to interfere with light reflected from adjacent marks, causing intersymbol interference (ISI). The effect of the ISI is greater when the marks are closer together. A xe2x80x9cmodulation transfer functionxe2x80x9d (MTF) describes the transformation of the detected read signal that results from interference from neighboring marks.
Optical data disc readers currently use analog filtering of the read signal to equalize the frequency response of the system. The equalization is an attempt to compensate for the MTF, which predicts how much contrast an optical imaging system will generate when scanning different spatial frequencies. Current art uses a simple frequency equalization as discussed in Principles of Optical Disc Systems, Bouwhuis, Braat, Huijser, Pasman, van Rosmalen, and Immink, 1985, Adam Hilger Ltd., Boston, Mass., chapter 2, which is herein incorporated by reference for all purposes.
Generally, the MTF does not have a flat frequency response; the magnitude of the MTF decreases monotonically with increasing spatial frequency, reaching zero at a limit called the optical cutoff frequency. For example, the peak-to-peak signal from a series of 0.83 xcexcm marks on a CD is approximately 40% that from 1.6 xcexcm marks. Above the cutoff frequency, which for a CD corresponds to 0.43 xcexcm marks, a CD reader would detect no contrast at all. Since the shorter marks correspond to higher temporal frequencies in the detector signal, one can xe2x80x9cequalizexe2x80x9d the contrasts of long and short marks by increasing the high-frequency gain in the reader electronics.
Digital equalization is superior to analog equalization, for various reasons as discussed in U.S. Pat. No. 5,818,806 (the ""806 patent). FIG. 1 is a block diagram which illustrates the multilevel optical data-storage and data recovery system using digital equalization described in the ""806 patent. A data signal 102 includes a sequence of multilevel symbols xi received from the modulation encoder. The value of each xi is one of M possible levels. Data signal 102 is fed to a precompensator 104 which precompensates the signal to help remove nonlinear ISI effects and possibly some linear ISI effects. The precompensated signal 106 is sent to an optical disc writing system 108, which writes to an optical disc 110. An optical disc reading system 112 then reads the disc. An analog-to-digital converter (ADC) 114 samples the optical read signal once per multilevel symbol. The sampled read signal 116 enters the digital equalizer 118, which inverts the uncompensated portion of the channel MTF, yielding recovered levels yi 120 that are identical to corresponding written levels xi plus some random noise component. Such an equalizer is called a zero-forcing (ZF) equalizer because, by inverting the channel MTF, it forces the ISI to zero.
A real system contains significant laser, media, and electronic noise that can result in errors in subsequent detection or decoding blocks. A ZF equalizer, by inverting the channel, boosts high frequencies of both signal and noise, thus enhancing the high-frequency noise components, which can lead to increased errors. Moreover, a filter with high-frequency gain has a relatively long time-domain response that requires many filter taps to implement, which leads to a high computational load. Also, in practical systems timing offsets in the ADC can significantly degrade the performance of baud-spaced equalizers.
In view of the foregoing, there is a need for methods for providing better equalization filters than those that are currently available. Specifically, it would be desirable if equalization filters could be developed which do not cause excessive noise enhancement during equalization and which have better immunity to fixed timing offsets in sampling.
Accordingly, a method is disclosed for providing improved digital equalization filters for multilevel data-storage that do not enhance high-frequency noise components, as well as filter structures that have superior performance to baud-spaced equalization filters, especially in the presence of timing offsets.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. Several inventive embodiments of the present invention are described below.
In one embodiment, a method of compensating for intersymbol interference in an optical data storage channel is shown. The method includes measuring an intersymbol linear modulation transfer function. Data is written to the optical data storage channel. A partial response target frequency response is divided by the intersymbol linear modulation transfer function for obtaining a linear equalization filter response. A read signal is obtained from the optical data storage channel. The read signal is convolved with the linear equalization filter response to produce an equalized read signal containing a controlled partial-response target intersymbol interference.
In another embodiment, a method of compensating for intersymbol interference in an optical data storage channel is described. The method includes reading a read signal from the optical data storage channel and equalizing the read signal to achieve a desired target response. The equalizing includes forward filtering the read signal to remove precursor intersymbol interference. The forward filtering produces a forward filtering output for slicing to produce tentative decisions. Reverse filtering the tentative decisions produces predicted postcursor intersymbol interference. Subtracting the predicted postcursor intersymbol interference from the forward filtering output thereby produces an equalized read signal.
In another embodiment, a method of compensating for intersymbol interference in an optical data storage channel is shown. The method comprises measuring an intersymbol modulation transfer function, determining a desired target response from the modulation transfer fiction, writing data to the optical data storage channel, reading a read signal from the optical data storage channel, and equalizing the read signal to achieve the desired target response. The equalizing includes forward filtering the read signal for removing precursor intersymbol interference, the forward filtering producing a forward filtering output, slicing the forward filtering output to produce tentative decisions, reverse filtering the tentative decisions to produce predicted postcursor intersymbol interference, and subtracting the predicted postcursor intersymbol interference from the forward filtering output thereby producing an equalized read signal.
These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example the principles of the invention.